Size reduction machines are widely used in the production of pharmaceuticals and cosmetics. The pharmaceutical or cosmetic is manufactured and then size reduced into a granular or fine powder form. The pharmaceutical and cosmetic industries have very strict sanitary standards for operation and production. Size reduction machines must be capable of being fully sanitized before it can be used in such environments.
Size reduction machines of the prior art utilize a frusto-conical shaped screen located in a channel between an input and an output. Such size reduction machines are more particularly described in U.S. Pat. No. 4,759,507. In these machines, various screens and impellers are used to reduce the size of the particles. The choice of screen and impeller depends on the size and type of product that is being processed. The screens can have apertures in different sizes and shapes to produce a desired milled product.
The size reduction machines of the prior art are driven by a motor operably connected to a shaft on which the impeller is mounted. The drive is transmitted normally by means of a plurality of belts. However, some size reduction machine have shaft drives. One such machine is described in German patent no. 36 17 175.
Regardless of the type of drive, heretofore the drive to the impeller has been applied to the shaft above the impeller. As a result, the input of material to be milled must be offset from the vertical axis of the machine. Hoppers and feed pipes must be used to direct the material from an input, about the drive and into the impeller region. The redirecting of material is inefficient and more importantly such prior art machines require more height than other types of mills.
The principal reason that the drive of a size reduction machine is mounted above the impeller is to remove the drive from the path of the material to be milled. By passing the material around the drive, the drive is outside of the sanitary region of the machine. These machines can therefore meet the industry standards for sanitation set by various regulatory bodies, including the Food and Drug Administration in the United States.
The physical size of the size reduction machine plays an important role when the size reduction machine is being installed in an existing material processing system as a replacement for another type of mill. If the size reduction machine is physically too large, it cannot be used as a replacement machine for other types of size reduction machines.
In German patent no. 36 17 175, the shaft drive is above the impeller and is fully enclosed permitting the drive to remain in the direct flow of the material to be milled. However since the material to be milled has very low kinetic energy before it enters the impeller region, the input to the impeller region can become jammed allowing the material to bridge over the input. A paddle is required to be mounted on the spindle to prevent such bridging of material.